In a particle beam therapy, a high-energy particle beam, such as a proton beam or a carbon beam accelerated up to 70% of the light velocity, is utilized. These high-energy particle beams have the following features when irradiated into a body. Firstly, almost all of irradiated particle beams stop at a depth position proportional to the particle beam energy raised to the 1.7th power. Secondly, the energy density (referred to as a dose), which is imparted to the irradiation path through which the particle beam passes until it stops in a body, becomes maximum at the particle-beam stop position. A distinctive deep dose distribution curve formed along a path through which a particle beam passes is referred to as “Bragg curve”. The position where the dose value becomes maximum is referred to as “Bragg peak”.
A three-dimensional particle beam irradiation system is contrived in such a way that, while it scans the Bragg peak position in accordance with the three-dimensional shape of a tumor and adjusts the peak dose at each scanning position, a predetermined three-dimensional dose distribution is formed in a tumor region, which is a target preliminarily determined by an imaging diagnosis. The scanning of the position where a particle beam stops includes scanning in transverse directions (X and Y directions) which are approximately perpendicular to the irradiation direction of a particle beam and scanning in a depth direction (Z direction) which is the irradiation direction of a particle beam. In the transverse-direction scanning, there exists a method of moving a patient with respect to a particle beam and a method of moving the position of a particle beam by use of an electromagnet or the like; in general, the method utilizing an electromagnet is adopted. Scanning in the depth direction is performed only by changing the energy of a particle beam. There are two methods for changing particle beam energy. One method uses an accelerator for changing energy; and the other method uses an energy changing apparatus called a range shifter (including an apparatus referred to as “energy selection system” serving as an energy changing and analyzing unit) installed in a beam transport system or an irradiation system. A commonly employed method is a method that uses a range shifter (for example, Patent Document 1 and Patent Document 2).